Catenary geodesic lens antenna



y 1968 J. L. M FARLAND 3,383,691

- CATENARY GEODESIC LENS ANTENNA Filed Oct. 1, 1965 5 Sheets-Sheet 1 Jerry L. McFarland,

INVENTOR.

ATTORNEY.

y 1968 J. 1.. MOFARLAND 3,383,691

CATENARY GEODESIC LENS ANTENNA Filed Oct. 1, 1965 5 Sheets-Sheet 2 22 Flg. 3.

2 L AP my r 45 Fag. 4.

Jerry L. McFarland,

INVENTOR.

ATTORNEY.

y 1968 J. L. M FARLAND 3,383,691

CATENARY GEODESIC LENS ANTENNA Filed Oct. 1, 1965 5 Sheets-$heet 5 Jerry L. McFarland,

INVENTOR.

ATTORNEY.

United States Patent 3,333,691 CATENARY GEODESEC LENS ANTENNA Jerry L. McFarland, Fullerton, Calif, assignor to Hughes Aircraft Company, Quiver City, Calif., a corporation of Delaware Filed Oct. 1, 1965, Ser. No. 492,661 Claims. (Cl. 343-754) This invention relates to directional antennas and more specifically to those of the type generally known as parallel plate or geodesic antennas.

Parallel plate or geodesic antennas, as they are more commonly known, find application in many modern communications and radar systems. Of particular importance are geodesic antennas which are capable of diecting electromagnetic wave energy from a small feed source to a relatively large aperture in such a manner that the radiated wave energy adds coherently in only one direction of space. Antennas having this property are theoretically capable of perfect focusing and are frequently so termed. It is recognized, of course, that the description perfectly focusing is only theoretically accurate and that due to minor irregularities in the antenna structures arising in construction or by mechanical stresses to which the structures are subjected, departure from the assumed theoretical focusing behavior occurs.

It is a general object of the present invention to provide an improved geodesic antenna structure capable of substantially perfect focusing.

In the past, geodesic antennas have been utilized which are theoretically capable of perfect focusing. The so-called folded pillbox antenna is an example of such a structure. This antenna utilizes a parabolic reflecting surface in conjunction with at least three flat parallel conductive plates forming the waveguiding boundaries. Due to the inherent properties of the parabolic reflecting surface, however, such an antenna is characterized by its very limited scanning capability. That is, the direction of the radiated wave energy cannot be readily varied by changing the position of the feed element without seriously degrading the focusing capability of the antenna.

It is another object of the present invention to provide an improved, substantially perfectly focusing antenna structure having an increased scanning capability.

Geodesic antennas have also been utilized which, while not perfectly focusing, do provide a high degree of scanabi-iity. These geodesic antennas are generally termed figures-of-revolution antennas. As their name implies, these antennas generally comprise two intersecting pairs of parallel conductive sheets, the mean surfaces of which comprise a plane and a surface generated by a line revolved about an arbitrary axis. For relatively small aperture dimensions such antennas have enjoyed some degree of success. However, for aperture sizes much greater than 25 wavelengths their focusing ability is degraded appreciably.

It is therefore another object of the present invention to provide an improved geodesic antenna having a focusing ability which is substantially unaffected by aperture size.

In accordance with the principles of the present invention, these objects are accomplished in a geodesic antenna utilizing a unique geometric configuration. Specifically, the invention utilizes a pair of parallel conductive sheets in the shape of catenary cylinders. An additional pair of parallel conductive members which can comprise planar conductive sheets or planar conductive lips are disposed at an angle to the axis of the catenary cylinders. Each of the cylindrical conductive sheets intersects and is conductively bonded to one of the planar conductive members. The open edges of the planar conductive members de- "ice fine the antenna aperture which can be straight or curved. An antenna feed element is disposed. in the region of the axes of the cylindrical conductive sheets at the ends thereof opposite their intersection with the planar conductive members.

In order that the said invention may be clearly understood and readily carried into effect, it will now be de scribed with reference by way of example to the accompanying drawings, in which:

FIG. 1 is a pictorial illustration of one embodiment of the present invention;

FIG. 2 is a pictorial view of the mean surfaces of a geodesic antenna useful in explaining the principles of the present invention;

FIG. 3 is a developed view of the mean surface of the geodesic antenna shown in FIG. 2;

FIG. 4 is a pictorial view of another embodiment of the present invention; and

FIG. 5 is a pictorial view of yet another embodiment of the present invention useful at relatively low frequencies.

In the drawings, FIG. 1 is a pictorial view of a structure known generally as a geodesic or parallel plate antenna. The antenna of FIG. 1 is formed by two pairs of intersecting thin parallel conductive sheets. The sheets 10 and Ill comprising one pair are of cylindrical shape. Sheets 12 and 13, which comprise the intersecting pair, are planar. Sheets 10, 11, 12 and 13 are all fabricated of a material such as copper having a high electrical conductivity at microwave frequencies. Cylindrical sheet 10 and intersecting planar sheet 12 are conductively joined along their common edge such as by soldering or welding. In a similar manner cylindrical sheet 11 and planar sheet 13 are conductively joined along their common edge.

The interspace between parallel sheets 18 and 11 and 12 and 13 is generally filled with a homogeneous dielectric medium such as air, although other low-loss dielectric materials can be utilized. In order to eliminate the possibility of unwanted higher order modes, the spacing between conductively joined intersecting sheets 10 and 12 and intersecting sheets 11 and 13 is generally less than onehalf wavelength at the highest frequency of operation, and is maintained by suitable spacers well-known in the, art.

In operation, wave energy from a source, not shown, is fed into the antenna by a section of waveguide 14. Feed waveguide 14 can extend a short distance into the interspace between cylindrical conductive sheets 10 and 11. The wave energy supplied by feed waveguide 14 is radiated from the aperture 15 formed by the opening between planar conductive sheets 12. and 13 as shown by arrows 16.

Of course, when the antenna is utilized for the reception of wave energy, waveguide section 14 serves to couple the antenna to a receiving circuit. In either event, such antennas are generally characterized by the fact that wave energy is propagated principally in the TEM mode between the parallel conductive sheets between the feed and the aperture. For the sake of analysis, rays such as those shown at arrows 16, can be imagined from the feed to each region of the aperture. These rays, which correspond to the paths of the propagating wave energy, can be thought of as lying in a surface midway between cylindrical sheets It) and 11 and planar sheets 12 and 13. In accordance with Fermats principle, these rays represent the shortest distance between the feed and the corresponding regions of the aperture.

Ideally, for perfect focusing, the phase centers of all wave energy emerging from the aperture should be collinear. This, in turn, generally requires that the total path lengths of all rays from the feed to the aperture be equal.

For lateral positions of feed waveguide 14 other than that shown (that is, for positions other than midway along the lateral arc of the cylindrical sheets) this requirement is desirable although not generally met. In order to obtain some degree of beam scanning with such an antenna, however, feed waveguide 14 is generally adapted for lateral motion between cylindrical conductive sheets 10 and 11.

As mentioned hereinabove, the so-called folded parallel plate parabolic pillbox antenna is an example of an antenna having a theoretically perfect focusing capability. That is, the radiated wave energy has a substantially collinear phase front. This antenna, however, is characterized by its very limited scanning capability. In the analysis set forth hereinbelow, a family of antennas is disclosed, the members of which are characterized by their perfect focusing and increased scanning capability.

In FIG. 2, there is shown a pictorial view of the mean surface of a geodesic antenna similar to that depicted in FIG. 1. In the figure, a curved cylindrical surface 2t) having an axis extending in the z direction is intersecte at a right angle by planar reference surface 21. In the coordinate system selected, reference surface 21 lies in the .ry plane normal to the z axis. A second planar surface 22 defined as lying in the xy plane passes through the origin 0, the x axis and cylindrical surface 20. The intersection of planar surface 22 and cylindrical surface 20 is shown by dashed line 23. The angle between planes 21 and 22 is designated at and therefore,

y'=y sec (1) z=y n For the sake of analysis, it is further proposed that the feed point of the antenna is situated at point P having coordinate values of x 0, y=0 and 2:). A typical ray r is shown connecting feed point P to the antenna aperture in the xy' plane. The point at which ray r intersects the common line 23 joining cylindrical surface 20 and planar surface 22 is designated R. Point R, for the sake of analysis, is assigned the generalized coordinates (x, y, z). The distance between the origin 0 and point R along cylindrical surface 20 is s, where,

In a like manner the projection of point R on the xy plane is designated R and the distance along cylindrical surface 20 to the origin 0 is designated s, where,

Having thus established the basic symbols relating to the geometry of the geodesic antenna, the present invention can now be analyzed in greater detail. For this purpose it is helpful to consider the developed view of FIG. 3.

Since the structure of FIG. 2 represents a singly curved structure, it can be developed as shown in FIG. 3 wherein like symbols have been carried over from FIG. 2 to represent like elements and dimensions. In the developed view of FIG. 3, cylindrical surface 20 has been flattened and folder so that it is coplanar with surface 22. The length of ray r from feed point P to point R is designated I and is given by:

Similarly, the distance I along ray 1- from point R to the aperture (at y' y is given by:

and

From Fermats principle mentioned above, the total path length of any ray such as r from the feed point P (18 (f+. +(f 1 2155 (f+y) (f- And by differentiating Equation 4 with respect to x,

ds dy) dx t rlx (10) By equating Equations 9 and 10 and substituting Equations 1 and 2, it is found that:

F6 W+2 y where 6 is given by the expression:

'Ozflsec o-l-tan a5) (12) Integrating Equation 11 yields:

1L a i a 00.11 1

Equation 13 is, therefore, the expression describing the shape of cylindrical surface 20 for perfect focusing. It is seen that cylindrical surface 20 assumes the shape of what is termed a catenary cylinder. Thus, for every value of q: (or '5), there is a catenary cylinder yielding a perfectly focusing antenna. All members of this family of antennas comprise separated parallel conductive sheets, the mean surfaces of which are formed by a catenary cylinder symmetrically insersected by a plane.

As mentioned above, in connection with the antenna of FIG. 1, the radiation from a geodesic antenna takes place at the aperture formed by the planar parallel conductive sheets 12 and 13. It is apparent, however, that radiation can take place nearer the intersection of the planar sheets and the cylindrical sheets. Such an arrangement may also be advantageous from the standpoint of increased directivity in the plane perpendicular to the planar intersecting sheets.

An embodiment so designed from this standpoint is shown in the pictorial view of FIG. 4. The antenna of FIG. 4 comprises a pair of parallel conductive sheets 40 and 41 in shapes conforming to catenary cylinders. The construction lines of the catenary cylinder comprising the mean surface between cylinders 49 and 41 is shown as surface 42. The mean surface of the intersecting planar conductive members is shown as construction plane 43. The intersecting planar sheets have been replaced by a pair of conductive projections or lips 44 and 45, each of which is conductively joined along its intersection to eatenary cylindrical sheets 40 and 41 respectively.

The embodiment of FIG. 4 is similar to that of FIG. 1 except that the surplus material has been trimmed from catenary cylindrical sl eets 40 and 41 and the planar conductive sheets have been replaced by planar conductive lips 44- and 45. As before, the parallel conductive members comprising the antenna are spaced apart a distance less than one-half wavelength at the highest frequency of intended operation. For the sake of clarity, the spacing means have been omitted from the drawing but can comprise any suitable low-loss spacers or supports well-known in the art. A feed structure such as waveguide 46 is provided for coupling outgoing wave energy to and incoming wave energy from the antenna. As mentioned above, waveguide section 46 can be adapted for lateral motion to provide a measure of beam scanning.

It is known that a uniformly loaded rope or cable suspended at two points along its length naturally describes a catenary curve. In keeping with the present invention, this property of suspended cables can be exploited to realize a large direction antenna especially suited to relatively low-frequency operation. Such an embodiment is shown in the pictorial view of FIG. 5.

In FIG. 5 a plurality of structures such as towers 50 provide support for cables 51. Extending between cables 51 are a pair of substantially parallel conductive sheets 52 and 53 each comprising, for example, a mesh or screen of conductive wire or cable. In the present sense the term sheet is used more for the sake of consistency than for semantic accuracy. The use of the term is, however, justified when it is recognized that a conductive screen will act as a reflecting surface for Wave energy having a wavelength much greater than the spacing between the conductors making up the screen.

A pair of protruding lips 54 and 55, which can also comprise a conductive screen, are provided for polarizing and directing the radiated wave energy in the desired direction. Lip 54 is conductively attached along its length to conductive screen 52 and lip 55 is conductively connected in a like manner to screen 53. Lips 54 and 55 can also be suspended from cables between two end towers 56 by suitable mechanical extensions.

A low-frequency directional radiating element or array 56 is disposed between screens 52 and 53 at the end thereof opposite lips 54 and 55 can comprise the feed element for the embodiment of FIG. 5. This feed element is not shown in detail but can comprise, for example, a directional radiating antenna such as a dipole with a quarter wavelength back reflector.

In all cases it is understood that the above-described arrangements are illustrative of but a small number of the many specific embodiments which can represent applications of the principles of the present invention. Numerous and varied other arrangements can be readily devised in accordance with these principles by those skilled in the art without departing from the spirit and scope of the invention.

What is claimed is:

1. A geodesic antenna comprising, in combination, two pairs of uniformly spaced conductive sheets, the mean surface of said pairs comprising a catenary cylinder intersected by a plane, one sheet of each of said pairs being conductively joined along the line of intersection with one sheet of the other of said pairs, and means for cou- 6 pling electromagnetic wave energy to and from said antenna.

2. An antenna of the class comprising two intersecting pairs of uniformly spaced conductive sheets, said antenna characterized by two sheets comprising a first of said pairs in the shape of catenary cylinders and two sheets comprising the other of said pairs in the shape of planar surfaces.

3. In combination:

a first pair of sheets of conductive material in the form of substantially parallel catenary cylinders;

a second pair of sheets of conductive material in the form of substantially parallel planes, said second pair of sheets being disposed at an angle to the axes of said first pair:

each sheet of said first pair intersecting and conductively joined to a respective sheet of said second pair; and

means for coupling electromagnetic wave energy to the interspace between said first pair of sheets at the end region thereof opposite said intersection.

4. A geodesic antenna comprising, in combination:

first and second pairs of uniformly spaced conductive surfaces;

a first surface of said first pair intersecting and conductively bonded to a first surface of said second pair;

a second surface of said first pair intersecting and conductively bonded to a second surface of said second pair;

said first pair of surfaces having shapes corresponding to catenary cylinders; and

said second pair of surfaces having shapes corresponding to planes.

5. In an antenna having a feed and an elongated radiating aperture, means for illuminating said aperture with electromagnetic wave energy having a substantially linear phase front, said means comprising a pair of serially connected parallel plate waveguides extending between said feed and said aperture, a first of said waveguides having a mean surface described by a catenary cylinder and the second of said waveguides having a mean surface described by a plane.

References Cited UNITED STATES PATENTS 2,653,239 9/1953 Lan Jen Chu et a1. 343-783 ELI LIEBERMAN, Primary Examiner. 

1. A GEODESIC ANTENNA COMPRISING, IN COMBINATION, TWO PAIRS OF UNIFORMLY SPACED CONDUCTIVE SHEETS, THE MEAN SURFACE OF SAID PAIRS COMPRISING A CATENARY CYLINDER INTERSECTED BY A PLANE, ONE SHEET OF EACH OF SAID PAIRS BEING CONDUCTIVELY JOINED ALONG THE LINE OF INTERSECTION WITH ONE SHEET OF THE OTHER OF SAID PAIRS, AND MEANS FOR COU- 